The composition of the pyrolysis products of some oligodimethylsiloxanes with cyclic and linear structures

THE COMPOSITION OF THE PYROLYSIS PRODUCTS OF SOME OLIGODIMETHYLSILOXANES WITH CYCLIC AND LINEAR STRUCTURES* ~/~. V. SOBOLEVSKII, I. I. SKOROKHODOV, V. YE. DITSENT, L. V. SOBOLEVSKAYA and G. M. MOISEYEVA (Received l0 September 1969)

THE main direction of the thermal degradation of polydimethylsiloxane (PDMSi) is known, at least in the usual range of experimental temperatures (up to 500°C); it consists of a siloxane bond rearrangement resulting in decomposition to low molecular weight (mol.wt.) products with a cyclic [1, 2], or cyclic and linear structure [3], depending on the type of terminal groups present. The mechanism of this important process, which to a large degree determines the heat resistance of the organosilicon polymers [4], is still unclear. In this context a study of the type of siloxane bond rearrangement during heating is of greater interest, using the separate dimethylsiloxanes as example, because one would expect many details of this conversion to become clearer here than in the polymer systems. This communication reports the results obtained during the pyrolysis of some oligodimethylsiloxanes of cyclic and linear structure, using the pyrolysis product composition of hexamethylcyclotri- (Ds) to dodecamethylcyclohexa- (De) siloxane, and from hexamethylcyclodi- (M2) to hexadecamethylhepta- (M2I)5) siloxanc respectively. EXPERIMENTAL The original dimethylcyclosiloxanes were extracted from a hydrolytic product mixture of dimethyldichlorosilane condensation, the dimethylsiloxanes of linear structure from the mixture of the catalytic rearrangement products of octamethylcyclotetrasiloxane with hexamethyldisiloxane in the presence of " K i l " clay. The lastnamed was isolated from the products of trimethylchlorosilane hydrolytic condensation. The chromatographic analysis showed all the substances to be of 98.0-100% purity (Table 1). The dimethylsiloxanes were pyrolysed in sealed " P y r e x " glass ampoules of about 20 ml volume, which had been filled with 1 g of the substance. Traces of air and moisture were removed by subjecting the dimethylsiloxanes to a 10-4-10 -5 m m H g vacuum for 3 hr before sealing the ampoules. The latter were then kept in a thermostatted oven at 400, 450 and 500-4- I°C for 5 hr. Visual observations showed pyrolysis to take place in the gas phase under these conditions (in all cases). The composition of the pyrolysis products was determined by chromatography [3] on an instrument first standardized for all the expected components. * Vysokomol. soyed. A12: No. 12, 2714-2720, 1970. 3082

Composition of pyrolysis products of some ohgodimethy|siloxanes TABLE

1.

CHROMATOGRAPHIC

AI~ALYS1S

RESULTS

ON

3083

STARTING

MATERIALS

L Cyclic siloxanes

i Content of main w/w ]substance, (~o

Linear siloxanes

99.9 99.4 98.1{ 99-7

MoD M~D2 M~D3 M~D~ M~D5

M~

D3

D4 D~ D6

* Traces: 1% D~, 0"5% M2I); 1 o/ /o I)~.

Content of main

!,~ut~stance, % w/w 98.0* 99.0t 99-6 99.6 100.0 99.7

t 0"3% 1%, 0"2% Da, 0"3 I) 4. 0"2% 5121)2; ~ | %

1)4,

RESULTS

The pyrolysis of dimethylcyclosiloxanes (DMCSi). The analytical results of the pyrolysis products are contained in Table 2. The first thing noticed was the specific behaviour in the hexamethylcyclotrisiloxane (HMCTSi) pyrolysis. This substance was almost completely converted at 400 and 450°C to polymers with mol.wt, of 1810 and 687 respectively; it did not polymerize at higher temperatures (500°C). Such pyrolysis conditions caused the production of a mixture of cyclic dimethylsiloxanes, in which octamethylcyclotetrasiloxane (OMCTSi) was dominant (56%), but it also contained 20% of HMCTSi. It was also interesting that the cyclic trimer pyrolysis products had the same composition at 500°C as the thermal degradation products of ~,og-hexamethyl-PDMSi of mol.wt. 2000 which had been subjected to the same treatment. No polymeric pyrolysis products were detected amongst the other dimethylcyclosiloxanes studied; cyclic siloxanes with a smaller mol.wt, were mainly produced at all temperatures (Table 2). For example, the main pyrolysis products obtained from OMCTSi, especially at 400 and 450°C, was the cyclic trimer. It is true to say that more and more of the cyclic pentamer was produced on raising the pyrolysis temperature. In the case of the decamethylcyclopenta- and dodecamethylcyclohexasiloxanes, cyclic trimers and tetramers were the main pyrolysis products. Their relative contents became similar on raising the temperature. The formation of cyclosiloxanes with larger mol.wt, than the original was not detected in these two cases. The comparison of the pyrolysis resistance of the examined DMCSi is also interesting, because it seems to be a function of the quantity of original material used. Table 2 shows the resistance of the cyclic tetramer, pentamer and hexamer to be practically the same at 400°C (at 1.5-1.8% conversion). Similar conversion capacities applied at 450°C only to the tetra- and pcntamer, i.e. 9.9 and 11.4°/o respectively under these conditions, while the hexamer showed an 18.7~o re-

M.V. SOBOLEVSKIIet al.

3084

TABLE 2. COMPOSITION OF THE DIMETIIYLeYCLOSILOXANEPYROLYSIS PRODUCTS Original

Temper-

compound

ature °C

Ds

D4

D~

D6

Pyrolysis products, % w/w D3

])4

D~

De

400 450 500

19"7

Polymer with mol.wt. 1810 Polymer with mol.wt. 687 56"6 18.4 4.6

400 450 500

1.8 7.7 15.4

98.5 90.1 77.9

0.1 6.0

0.5 0.7

400 450 500

0.9 7.6 20.7

0.2 2.8 25.0

98.4 88"6 54.O

0.6

0.1

400 450 500

1.1 ll.3 21.4

0.4 5.0 19.8

0.1 1.8 8.9

98.2 81.7 49.6

0"0 0.4 0.7

1.7

D7

0.9

arrangement during pyrolysis. In contrast with this, the reactivities of the pentaand hexamer were comparable at 500°C, namely 46.0-50.00/o conversions, while the tetramer remained more resistant, giving only 22% conversion. To find an explanation for these experimental findings, it was necessary to assume from the start that the activation energy required for the HMCTSi ring to open is much smaller than that required to open the less conjugated larger rings, and also that for depolymerization of the polysiloxanes to rings. Provided that this theory is accepted, one can assume the primary stage of the HMCTSi pyrolysis to be ring-opening, CH3

CH8

CH8

I

i

I

i

]

i

D3-*'Si--O--Si--O--Si--O', CH3

CH8

(1)

CHs

this being followed by rapid polymerization of the cyclic trimer to CH3

CH3

CH3

CH8

CII3 i

I I l i "Si--O--Si--O--Si--O'+ D~--*'Si-I CH3 CH3[,

I CH3

I CH8

CIt3],

"Si-- --O--Si

I

/

CH, L

I

__O__S~i__ l|/-Si-O"

i CtI3

CH3,

I Ctt8

CHs[,

--Si--O'+D,~'Si--

/

CHsJ¢

I

CHs

I

CH8

J, c/H~

|CH3] --O--Si--

/

CH, L

I

(2)

CH3 --Si--O"

/

CHsJ7

(3)

I

CH8

etc. the r a t e of d e p o l y m e r i z a t i o n , ac cording to formulae ( 1 )-(3), of the polysiloxane chains a t 400 a n d 450°C will t h u s be small; pyrolysis will cause ~lmost complete

Composition of pyrolysis p r o d u c t s of somo oligodimethylsiloxanes

3085

polymerization of the Original trimer in the given pyrolysis time. The rate of depolymerization to rings at 500°C seems to exceed that of chain formation, so that there is no polymer accumulation, and the pyrolysis products are only rings with 4-7 Si-atoms. The earlier noted compositional similarity of the pyrolysis products obtained from the cyclic trimer with that of thermal degradation of the ~49-hexamethylpolysiloxane at 500°C agrees well with the developed theories, and can be regarded as confirmation of their validity. The primary stage of ring opening is also valid for the pyrolysis of other DMCSi: CH3

H31

r

D,~ - + ' S i - - - - O - - S i - - /

CH3

I

--Si--O"

(4)

CIH~J ,~-2 CHa I

CH3

because of the larger resistance of the rings, there is a strong resistance to polymerization following formulae (2)-(3). Instead, the cnergeticMly more favourable siloxane bond rearrangement in the rings during the reaction with the biradicals, produced according to formula (4), plays the important part.

/ --si--o' ,

• CH 3 L

CHa]~

/ CH 3 L

CH3

- - S i - - O ' -]- D E~ 2D3 + D4

(6)

I

CH3 j a CH3

CH3

CH3

CH3

I

r

I

I

I

I

"Si - - O - - S i - - O - - S i --O" -t- DE -* 2D,~ CH~

CH3

olo,i

/%,, OH3 I

CH3 f

OH3 I

CH~ I

" S i - - O - - S i - - O - - S i - - O " 4- D0-~D4-~- D5 CI-[ 3

I

CH 3

(7)

CH3

"Si--/--O--Si--/ --Si--O" + I),-.D3+ D, + D~

I

(5)

(S)

(9)

$

CHz

etc. A dominance of the above processes over a polymerization of the type shown by formulae (2)-(3), during the pyrolysis of cyclic siloxanes Dn>~4, in addition to the energy factor, will be of an entropic nature [6]. It is quite clear that scheme (4)-(9) agrees well with the experimental findings in the case of the tetramer, pentamer etc., and explains the observed contraction of the original rings, as well as their enlargement. We had said earlier that the trimer does not show any polymerization tendency during pyrolysis at 400 and 450°C, although

3086

M.V. SOBOLEVSXIIet al.

hexamethylcyclosiloxane is completely converted to the polymer at these temperatures. This fact can be explained by assuming that the polymerization of the trimer by the scheme outlined in formulae (2)-(3) in the presence of other, more resistant DMCSi, is less probable than in a reaction of the respective biradical with rings of the type shown by formulae (7), (9), etc. The pyrolysis of linear dimethylsiloxanes. The composition of the pyrolysis products is shown in Table 3. These results indicate hexam~thyldisiloxane to be the most heat-resistant; its pyrolytic conversion did not exceed 5%, even at 500°C. The main product was octamethyltrisiloxane. This is a very interesting result, because it can serve as an indication of the homolytic nature of the siloxane bond fracture; the production of a linear trimer in this process is more naturally explained by: CHa

CHa

CH3

I

CH3

i

i

i

I CH3

I CH3

c /,H

i CH8

CI-I~

CH3

CH3

Ctt8

J

i

I

I CH8

I 0Hs

c/H,

CHs--Si--O--Si--CH3 --*CH3--Si" -4-"O--Si--CI-I3

J

CHa--Si--O--Si--CHa--*CHa--Si--O--Si'A-CHs"

I CH3

(10)

(i 1)

and CH3

CH8

t

I

CH3

I

I

I

I

CH~--Si--O--Si' A-'O--Si--CH3 -*M2D CH3

CH3

( 12}

CI-Is or

CH3

/

CH8

CH~

I

I

CH8

CH3

CHa --Si - - 0 --Si --CH3 A-'O --Si --CH3 -~MaD + "CH3

( 13~,

CH3

i

CH3--Si' ~- "CH3-, Si(CH3)4

(14)

I

CH3

Linear tetra- and pentasiloxancs were also detected amongst the pyrolysis products of hexamethyldisiloxane. Although their formation could be explained also by scheme (10)-(14), these are more likely to be the results of secondary reactions of the already produced octamethyltrisiloxane (see below). Changing now to the oligomeric homologues of hexamethyldisiloxane, one finds that there is a steady decrease of resistance where it was assessed on the ba§is of the detected amount of original compound, as was done previously in the case of the cyclic dimethylsiloxanes. For example, the conversion was 17%

Composition of pyrolysis products of some oligodimethylsiloxancs TABLE

3.

COMPOSITION

OF

THE

PYROLYSIS

PRODUCTS

OBTAINED

FROM

LINEAI~

3087 DIMETHYL-

SILOXANES -

r=-

=

P y r o l y s i s pro(tucts, % w / w

Original

[

COlll-

M2

pound

M2D

M2D2

M2D3

!

M2D 4 M2D5

D3

D4 i

0-7 0.3 0.9

0"3 0"3 0'7

0"1 0'7 1.2

0"1 0"6 1'5

D5

D,~

0"2 1"1 3'4

0"4

0.2

J

M,~D

M~D2

M~D3

M..,D4

M,.,D5

400 450 500

98.8 97-8 95.2

0.6 1.3 3.6

0-2 0"2 0-5

0'2 0"1

400 450 500

0.9 1.5 7.1

96.8 92.5 82.9

0.6 4.4 6.9

0.1 0.5 1.4

0.2

400 450 500

0.1 0.9 3.3

0.2 2.8 7.5

98.8 89.6 77.4

0'2 3-0 5'5

1.5 2'7

400 450 500

0.3 3.2

0.5 3.6 I 5.1

0'6 3.8 6-9

97.1 86.7 72.1

0.4 1-4 4.0

1.7

0'7 2.8 3.4

4O0 450 5OO

0.3 1.0 4'1

0.4 3'0 5'6

0'6 4.8 5.7

0.6 4.8 6.1

94.5 75.6 56'9

1.5 4.6 5-2

1.1 2.5 5.8

0"4 3"7 8'4

0"4 1 "5

0-2 o.5

40O 450 500

0.1 1.5 1.6

0'3 2-0 3"8

0"5 3.1 5"5

0-5 2"8 6'7

0.8 3.8 8.2

95.8 77.6 59.0

1-0

5"0 8.2

0'5 3"4 5'5

0"4 1 "4

(1.2

1.0

-

-

0"3

--

* In addition to the products shown above, 0.2-0-5% w/w of compounds were not identified.

under the severest conditions at 500°C in the case of the linear trimer, 22.6% with the tetramer, 28% with the pentamer, and around 41-43% with the hexaand heptamers. The effect of the molecular length of the siloxane on its heat resistance seems to become steadily weaker. An increase in chain length to 15-18 dimethylsiloxy units did not reduce the heat resistance of the dimethylsiloxanes [3]. The pyrolysis products of the linear trimer showed a dominant content of hexamethyldi- and decamethyltetrasiloxanes. There was an additional small quantity of dodeeamethylpentasiloxane and of cyclic tri- and pentamers present. An explanation of such a composition makes it necessary to assume that the following reactions took place during its degradation: CH3

I

M~D-~M2+'Si--O"

I

(15)

CH3 2M2D --*Mz+ M~Dz

(! 6)

M. V. SOBOLEVSKII et al.

3088

CH, M,D,+‘Si-O’+M,D,

CH,

(17)

CH,

2’gi-_O’+’ JH,

CH,

Ai-0-Si-0’ CH,

CH,

CH,

I (18)

CI!I, CHs

I

.Si-0-$i-O’+‘f!i-O.-D,

CH,

CL,

etc.

(19)

dH,

The pyrolysis products of the decamethyltetraand dodecamethylpentasiloxane etc. were also found to contain linear and cyclic siloxanes of smaller and larger mol.wt. than the original compound. The pyrolysis picture seems to be complicated by processes involving the freshly produced molecules. It is thu, impossible to give an exact reaction scheme; one can only state that pyrolysis will take place, even in these cases, through the intermediate formation of M’s ‘D’, MD’, and also MD’,, MD’,, DD, etc., radicals, which will recombine to give all the products obtained in each case. CONCLUSIONS (1) The pyrolysis products of some of the oligodimethylsiloxanes with a cyclic and linear structure were examined. (2) The main process which takes place during dimethylcyclosiloxane pyrolysis was found to be a ring contracting. (3) The pyrolysis of linear dimethylsiloxanes was found to result in the formation of linear and cyclic siloxanes. (4) General reaction schemes are given to describe the pyrolysis of the di-

methylsiloxanes. Translated by K. A: ALLEN REFERENCES 1. W. PATNODE

and D. F. WILCOCK,

2. D. C. ATKINS,

C. II. MURPHY

3. M. V. SOBOLEVSKII,

J. Am. Chem. Sot. 68: 358, 1946

and C. E. SAUNDERS,

I. I. SKOROKIIODOV,

and Ye. M. YEFIMOVA,

Vysokomol.

soyed.

Ind. Eng. Chem. 39: 1395, 1947

V. Ye. DITSENT, All:

Sci. U.S.S.R. 11: 5, 1257, 1969) 4. F. STONE and V. GRAHAM, In: Inorganic Polymers,

1109,

L. V. SOBOLEVSKAYA

1969 (Tr~nslsted

Izd. “Mir”,

in Polymer

1965 (Russian transla-

tion) 5. B. M. LUSKINA, G.

N.

V. D. MERKULOV,

TURKEL’TAUB,

Gazovaya

N. A. PALAMARCHUK, khromstografiya

illsed. inst. tekh. khim., No. 7, 112, 1969 6. A. B. BURG, J. Chem. Phys. 37: 482, 1960

(Gas

S. V. SYAVTSILLO Chromatography).

and

Nauch.